High moisture absorbent nonwoven fabric structure and production method thereof

ABSTRACT

A high moisture absorbent nonwoven fabric structure comprising a moisture absorbing layer mainly consisting of high moisture absorbent fibers, and surface sheets sandwiching the moisture absorbing layer to constitute a three-layer structure. The high moisture absorbent fibers gel on the surfaces thereof by absorbing water and/or moisture to develop a self-bonding force. The high moisture absorbent nonwoven fabric has an absorptivity, after five hours at 20° C. and relative humidity of 65%, of at least 20 wt. %, at least one of the surface sheets has breathability, and the self-bonding force of high moisture absorbent fibers ensures bonding between fibers in the moisture absorbing layer and between the moisture absorbing layer and the surface sheets. The high moisture absorbent nonwoven fabric structure, which is high in hygroscopicity, applicable to a small space, and easily recyclable by heat treating nonwoven fabrics after moisture absorbed, can be used in a variety of fields such as desiccants and dehumidifying agents.

TECHNICAL FIELD

The present invention relates to a high moisture absorbent nonwovenfabric structure. More particularly, it relates to a high moistureabsorbent nonwoven fabric structure which has high moisture absorptioncapacity and is suitably used as drying agents used in transportation ofprecision parts, dew condensation preventing agents for industrialequipments, drying agents for eliminating moisture in bags forconfectioneries, demoisturizers and the like, and its preparation.

BACKGROUND ART

As electronic parts such as silicon wafers cannot be used even whenslightly rusted, close attention is paid for their transportation.Electronic parts are put in a tightly closed container together with adrying agent for transportation as they are apt to rust when thehumidity comes higher. As the drying agent, silica gel is mainly used atpresent. Though silica gel can be recycled by heat treatment, it hasproblems of heat treating cost and breakage of particles during heattreatment. Hence, the silica gel drying agent is usually discarded afteruse. Though incineration or reclamation is used as the discardingmethod, the treatment shall be performed in a large amount and variousproblems such as incineration cost or lack of land to be reclaimed ariseand thus materials easily reusable are desired.

Though silica gel has been used for dew condensation prevention ofindustrial equipment in almost all cases, silica gel has a disadvantageof bulkiness as it is used in particle form. On the other hand,miniaturization of dew condensation inhibitor is desired accompanied tominiaturization of equipment.

Silica gel and deliquescent salts such as calcium chloride are used asthe drying agent and dehumidifying agent for confectioneries. Althoughthey are disposed as non-inflammable refuse in average home, there areproblems such as lack of land to be reclaimed and thus reduction inquantity is desired. Further, deliquescent salts become liquid bymoisture absorption and are in danger of leaking from packing papers andin addition in danger of generating heat when water is added to cause aburn.

On the other hand, the moisture absorbent nonwoven fabric is commonlyprepared by a procedure in which high moisture absorbent fibers aremixed with other synthetic fibers and/or natural fibers and carded andneedle-punched. However, the high moisture absorbent fiber is low infiber strength and fibers are fell off highly during the manufacturingprocess at a higher mixing rate and further twine round the machine asthey absorb moisture during the manufacturing process disadvantageously.Particularly, the high moisture absorbent fiber of the type wherein thesurface gels by absorbing water and/or moisture has low fiber strengthand the mixing rate is limited to about 40 weight %.

Other methods for the preparation of moisture absorbent nonwoven fabricsinclude air laid method. However, in this method, it is required to mixheat fusion welding fibers to adhere the fibers with each other or toadhere the fiber layers with the surface sheets and also it is requiredto increase the mixing rate of heat fusion welding fibers for keepingthe shape.

The present invention has been made in consideration of the abovecircumstance and its object is to provide a high moisture absorbentnonwoven fabric which can be easily recycled after use and itspreparation method.

DISCLOSURE OF THE INVENTION

We, inventors, have eagerly investigated to solve the above problem and,as the results, have invented the high moisture absorbent nonwovenfabric of the present invention. Thus, the high moisture absorbentnonwoven fabric structure of the present invention is a high moistureabsorbent nonwoven fabric structure consisting of a three layersstructure comprising a moisture-absorbing layer consisting mainly ofhigh moisture absorbent fibers and surface sheets with themoisture-absorbing layer between and it is characterized in thefollowing (a) to (f).

(a) Surface of said high moisture absorbent fiber gels by absorbingwater and/or moisture to express self-adhering activity.

(b) The fibers in the moisture-absorbing layer adhere each other and themoisture-absorbing layer and the surface sheets adhere each other by theself-adhering activity of the high moisture absorbent fiber.

(c) At least one of the surface sheets has air permeability.

(d) The thickness of the high moisture absorbent nonwoven fabricstructure is not more than 1.5 mm.

(e) The water content of the high moisture absorbent nonwoven fabricstructure is 20 to 45 weight %.

(f) The moisture-absorbing rate of the high moisture absorbent nonwovenfabric structure is at least 20 weight % under absolute dry condition at20° C. and 65% RH after 5 hours.

A preferred embodiment of the high moisture absorbent nonwoven fabricstructure is a high moisture absorbent nonwoven fabric structure inwhich the moisture-absorbing layer consists only of high moistureabsorbent fibers. More preferably, the high moisture absorbent fibersare of a crosslinked sodium acrylate type.

The apparent density of the high moisture absorbent nonwoven fabricstructure is preferably 0.05 to 0.8 g/cm³.

Furthermore, the high moisture absorbent nonwoven fabric structurehaving a three layers structure is preferably prepared through amoistening step and a drying step.

The present invention relating to the method for the preparation of ahigh moisture absorbent nonwoven fabric structure is also characterizedin that a fiber mixture mainly containing high moisture absorbent fibersare scattered continuously on one surface sheet having air permeabilityon a net while sucked from net side to form a moisture-absorbing layer,and then a moistening step is applied on the moisture-absorbing layer togel the surface of the high moisture absorbent fiber and then anothersurface sheet is laminated on it to give a three layers structure andthen it is pressed and dried to adhere the fibers in themoisture-absorbing layer each other and adhere the moisture-absorbinglayer with the surface sheets.

BEST MODE FOR CARRYING OUT THE INVENTION

The high moisture absorbent fiber used in the moisture-absorbing layerof the present invention should be such one that the surface gels byabsorbing water and/or moisture. Because, when the high moistureabsorbent fiber is such one that the surface gels by absorbing waterand/or moisture, self-adhering activity is expressed and the fibers inthe moisture-absorbing layer adhere each other and themoisture-absorbing layer and the surface sheet adhere each other by theself-adhering activity, and hence any thermal bonding fiber is notrequired and a higher mixing rate of the high moisture absorbent fiberscomes to be possible. When the high moisture absorbent fiber does notgel, self-adhering activity is not expressed and the fibers in themoisture-absorbing layer do not adhere each other and the moistureadhering layer and the surface sheet do not adhere each other and theshape cannot be kept.

The high moisture absorbent nonwoven fabric structure of the presentinvention shall be of a three layers structure comprising amoisture-absorbing layer and surface sheets with the moisture-absorbinglayer between. If the moisture-absorbing layer is exposed on thesurface, the high moisture absorbent fibers fall off during productionand no stable production can be made.

The material and the shape of the surface sheet used in the presentinvention are not especially restricted but it is required that a sheethaving air permeability is used in at least one side. As the sheethaving air permeability, for example, paper sheets such as tissue paper,cloths such as woven and knitted fabrics and nonwoven fabrics, andperforated non-air-permeable films can be used. As the non-air-permeablesheet, a film can be used. The material for surface sheet is preferablyone containing no harmful material such as chlorine from the viewpointof discarding after use.

When sheets having air permeability are used for the both sides of thesurface sheet, the moisture-absorbing rate is higher. When a sheethaving air permeability is used for one side, the moisture-absorbingsurface can be limited to one side. The moisture-absorbing rate can bevaried at will by varying properly the air permeability of theair-permeable sheet. When non-air-permeable sheets are used for the bothsides, the high moisture absorbent fibers come to not be able to absorbmoisture.

The thickness of the high moisture absorbent nonwoven fabric structureof the present invention is required to be not more than 1.5 mm. Whenthe thickness is as thin as not more than 1.5 mm, it can be used in asmall gap. It is preferably 0.2 mm to 1.2 mm, more preferably 0.3 mm to1.0 mm.

The moisture content of the high moisture absorbent nonwoven fabricstructure of the present invention is required to be 20 to 45 weight %.Though the nonwoven fabric structure is rolled for transportation andstorage, the nonwoven fabric structure comes rigid when the moisturecontent is lower than 20 weight % to cause cracking when rolled. On theother hand, when the moisture content exceeds 45 weight %, the nonwovenfabric structure comes heavier and softer and the thickness and theweight per unit area are in danger of being changed due to its dead loadduring transportation and storage. At a moisture content of 20 to 45weight %, it has proper pliancy and there is no danger of changing thethickness and weight per unit area. It is preferably 30 to 40 weight %.

Though the high moisture absorbent nonwoven fabric structure can be usedas it is, it is preferred to be used after dried to a favorite moisturecontent according to the purpose of use. The drying method is notparticularly restricted. For example, there may be exemplified methodsusing a hot air drier, a vacuum drier, a hand drier or the like.

The moisture-absorbing rate of the high moisture absorbent nonwovenfabric structure of the present invention shall be at least 20 weight %under an absolute dry condition at 20° C., 65% RH for 5 hours.Preferably, it is at least 30 weight % under the same condition. Whenthe moisture-absorbing rate is at least 20 weight %, a sufficientperformance can be attained as a drying agent or a moisture-removingagent.

Though the mixing rate of the high moisture absorbent fibers used in themoisture-absorbing layer of the present invention is not defined, it ispreferably at least 80 weight %. It is particularly preferred to be amoisture-absorbing layer consisting of high moisture absorbent fibersalone.

As the high moisture absorbent fiber in which the surface gels byabsorbing water and/or moisture, fibers of crosslinked sodium acrylatetype are exemplified. As the practical example, there is exemplified“BELLOASIS” made by Kanebo Gohsen, Ltd. The fiber has amoisture-absorbing rate of 40 weight % at 20° C., 65% RH and also hasexcellent properties of high moisture-absorbing velocity andmoisture-releasing velocity.

The high moisture absorbent fiber swells by containing water in a largeamount and the fiber surface gels to form mutually a pseudo-adhesionstate. Then, the self adhesion between high moisture absorbent fibers iskept by removing water by drying.

Also, the apparent density of the high moisture absorbent nonwovenfabric structure of the present invention is preferably 0.05 to 0.8g/cm³. More preferably, it is 0.15 to 0.5 g/cm³. At an apparent densityof 0.05 to 0.8 g/cm³, it is good in form-retaining and excellent inmoisture-absorption.

The high moisture absorbent nonwoven fabric structure of the presentinvention having three layers structure is preferably prepared through amoistening step and a drying step.

The moistening step means a step in which water and/or vapor is absorbedin the high moisture absorbent fibers to make gel of the surface of thehigh moisture absorbent fibers. When water is used in the moisteningstep, it is more preferred to give a sucking step immediately after themoistening step.

The drying step in the present invention means a step in which waterabsorbed in the moistening step is removed and the gel portion of thehigh moisture absorbent fibers are restored to the original condition toadhere the fibers in the moisture-absorbing layer mutually and adherethe moisture-absorbing layer with the surface sheets. As the dryingmeans, there are exemplified a heating furnace, a hot air drier andheating roller, and it is preferred to dry at 80 to 150° C.

Now, the method for the preparation of the high moisture absorbentnonwoven fabric of the present invention will be illustrated.

In the manufacturing method of the present invention, a surface sheethaving air permeability is placed on a net and a fiber mixture mainlycontaining high moisture absorbent fibers are scattered continuously onthe surface sheet while sucked from net side to form amoisture-absorbing layer. Then, water and/or vapor is absorbed(moistening step is applied) on the moisture-absorbing layer to inducegelation of the surface of the high moisture absorbent fibers. Then,another surface sheet is laminated on it to give a three layersstructure and then it is pressed and dried to adhere the fibers in themoisture-absorbing layer each other and adhere the moisture-absorbinglayer with the surface sheet to give a high moisture absorbent nonwovenfabric structure.

When water is used in the moistening step, it is preferred to carry outa step of sucking from lower side immediately after the moistening step.Or, it is preferred water is scattered from the lower side to improveadhesion to the lower surface sheet.

EXAMPLES

Now, the present invention will be illustrated in details by Examplesand Comparative Examples as follows.

The test method for moisture-absorbing rate is shown as follows.

(Test for Moisture-Absorbing Rate)

A sample was dried in a hot air drier at 120° C. for 2 hours and stoodin an air-conditioned chamber at 20° ° C. and 65% RH for 5 hours andthen the moisture-absorbing rate was calculated by the followingequation.Moisture-absorbing rate (%)=(W ₁ −W ₀)/W ₀×100W₀; Weight of the nonwoven fabric immediately after drying (g)W₁; Weight of the nonwoven fabric after moisture absorption (g)

Example 1

A super water and moisture absorbent fiber “BELLOASIS” of 10 dtex, 6 mmmade by Kanebo Gohsen, Ltd. was continuously scattered uniformly on atissue having a weight per unit area of 14 g/m² on a suction net by airlaid method and laminated to make a sheet of 200 g/m². Then, water wassprayed in the state of mist on the moisture-absorbing layer and then atissue of 14 g/m² was laminated on the moisture-absorbing layer to givea three layers structure. It was passed through a press roll to adjustthe thickness to 1.0 mm and then dried in a hot air drier at 130° C. Theresultant nonwoven fabric had a weight per unit area of 228 g/m², athickness of 1.0 mm, a moisture content of 38 weight % and an apparentdensity of 0.228 g/cm³.

The moisture-absorbing layer of this sample was adhered by self-adheringactivity of “BELLOASIS” and the moisture-absorbing layer was adhered tothe tissue of the surface sheet by “BELLOASIS”. The moisture-absorbingrate of the high moisture absorbent nonwoven fabric structure was 35weight %. The high moisture absorbent nonwoven fabric structure wasdried at 120° C. for 2 hours and the moisture-absorbing rate was againmeasured to be 35 weight %.

Example 2

The fiber “BELLOASIS” 10 of dtex, 6 mm was scattered continuously on aspunbonded nonwoven fabric “Eleves” of 15 g/m² made by Unitika, Ltd. ona suction net uniformly by air laid method and laminated to make a sheetof 150 g/m². Then, steam was blown on the moisture-absorbing layer and“Eleves” of 15 g/m² was laminated on the moisture-absorbing layer togive a three layers structure and it was passed through a press roll toa thickness of 0.7 mm and dried in a hot air drier at 130° C. Theresultant nonwoven fabric had a weight per unit area of 180 g/m², athickness of 0.7 mm, a water content of 30 weight % and an apparentdensity of 0.26 g/cm³.

The moisture-absorbing layer of this sample was adhered by self-adheringactivity of “BELLOASIS” and the moisture-absorbing layer was adhered tothe surface sheet “Eleves” by “BELLOASIS”. The moisture-absorbing rateof the high moisture absorbent nonwoven fabric structure was 33 weight%. The highly moisture absorbent nonwoven fabric structure was dried at120° C. for 2 hours and the moisture-absorbing rate was again measuredto be 31 weight %.

Example 3

The fiber “BELLOASIS” of 10 dtex, 6 mm was scattered continuously on“Eleves” of 15 g/m² on a suction net uniformly by air laid method andlaminated to make a sheet of 150 g/m². Then, water was sprayed in thestate of mist on the moisture-absorbing layer and a polypropylene filmof 20 g/m² was laminated on the moisture-absorbing layer to give a threelayers structure and it was passed through a hot press roll to athickness of 1.0 mm and dried in a hot air drier at 130° C. Theresultant nonwoven fabric had a weight per unit area of 185 g/m², athickness of 1.0 mm, a water content of 40 weight % and an apparentdensity of 185 g/cm³.

The moisture-absorbing layer of this sample was adhered by self-adheringactivity of “BELLOASIS” and the moisture-absorbing layer was adhered tothe surface sheet “Eleves” by “BELLOASIS” and the moisture-absorbinglayer was also adhered to the polypropylene film by “BELLOASIS”. Themoisture-absorbing rate of the high moisture absorbent nonwoven fabricstructure was 32 weight %. The high moisture absorbent nonwoven fabricstructure was dried at 120° C. for 2 hours and the moisture-absorbingrate was again measured to be 31 weight %.

Example 4

The fiber “BELLOASIS” of 10 dtex, 6 mm was scattered continuously on atissue of 14 g/m² on a suction net uniformly by air laid method andlaminated to make a sheet of 300 g/m². Then, water was sprayed in thestate of mist on the moisture-absorbing layer and a tissue of 14 g/m²was laminated on the moisture-absorbing layer to give a three layersstructure and it was passed through a press roll to a thickness of 0.8mm and dried in a hot air drier at 120° C. The resultant nonwoven fabrichad a weight per unit area of 328 g/m², a thickness of 0.8 mm, a watercontent of 39 weight % and an apparent density of 0.410 g/cm³.

The moisture-absorbing layer of this sample was adhered by self-adheringactivity of “BELLOASIS” and the moisture-absorbing layer was adhered tothe surface sheet “Eleves” by “BELLOASIS” and the moisture-absorbinglayer was also adhered to the tissue of the surface sheet by“BELLOASIS”. The moisture-absorbing rate of the high moisture absorbentnonwoven fabric structure was 36 weight %. The high moisture absorbentnonwoven fabric structure was dried at 120° C. for 2 hours and themoisture-absorbing rate was again measured to be 36 weight %.

Example 5

80 weight % of the fiber “BELLOASIS” of 10 dtex, 6 mm and 20 weight % ofconifer pulp were scattered continuously on a tissue of 14 g/m² on asuction net uniformly by air laid method and laminated to make a sheetof 200 g/m². Then, water was sprayed in the state of mist on themoisture-absorbing layer and a tissue of 14 g/m² was laminated on themoisture-absorbing layer to give a three layers structure and it waspassed through a press roll to a thickness of 0.6 mm and dried in a hotair drier at 120° C. The resultant nonwoven fabric had a weight per unitarea of 228 g/m², a thickness of 0.6 mm, a water content of 45 weight %and an apparent density of 0.380 g/cm³.

The moisture-absorbing layer of this sample was adhered by self-adheringactivity of “BELLOASIS” and the moisture-absorbing layer was adhered tothe surface sheet “Eleves” by “BELLOASIS” and the moisture-absorbinglayer was also adhered to the tissue of the surface sheet by“BELLOASIS”. The moisture-absorbing rate of the high moisture absorbentnonwoven fabric structure was 29 weight %. The high moisture absorbentnonwoven fabric structure was dried at 120° C. for 2 hours and themoisture-absorbing rate was again measured to be 28 weight %.

Comparative Example 1

50 weight % of the fiber “BELLOASIS” of 10 dtex, 6 mm and 50 weight % ofconifer pulp were scattered continuously on a tissue of 14 g/m² on asuction net uniformly by air laid method and laminated to make a sheetof 200 g/m². Then, water was sprayed in the state of mist on themoisture-absorbing layer and a tissue of 14 g/m² was laminated on themoisture-absorbing layer to give a three layers structure and it waspassed through a press roll to a thickness of 1.0 mm and dried in a hotair drier at 130° C. The resultant nonwoven fabric had a weight per unitarea of 228 g/m², a thickness of 1.0 mm, a water content of 35 weight %and an apparent density of 0.228 g/cm³.

Though the moisture-absorbing layer of this sample was adhered byself-adhering activity of “BELLOASIS”, the adhering activity was weak.Also, though the moisture-absorbing layer was adhered to the tissue ofthe surface sheet by “BELLOASIS”, the adhering activity was weak.Furthermore, the moisture-absorbing rate of the high moisture absorbentfabric structure was as low as 18 weight %.

Comparative Example 2

A super moisture absorbent fiber “N-38” of 4.4 dtex, 6 mm made by ToyoboCo., Ltd. was scattered continuously on a tissue of 40 g/m² on a suctionnet uniformly by air laid method and laminated to a web of 200 g/m².Then the web was stood at 30° C. and 90% RH for 10 minutes to, wet“N-38” and then a tissue of 40 g/m² was laminated on themoisture-absorbing layer to give a three layers structure. It was passedthrough a press roll to a thickness of 1.0 mm and dried in a hot airdrier at 130° C. “N-38” did not gel though moistened and no nonwovenfabric could be prepared.

INDUSTRIAL APPLICABILITY

The high moisture absorbent nonwoven fabric structure of the presentinvention contains the high moisture absorbent fibers in a high mixingrate and therefore it is high in hygroscopicity and can be used in asmall space as it is very thin. Furthermore, as it can be easilyrecycled by heat-treating the nonwoven fabric after moisturized, it canbe used in various fields including drying agents and dehumidifyingagents.

1-6. (canceled)
 7. A drying agent having a nonwoven fabric structure,comprising: a) the nonwoven fabric structure having three layerslaminated in order, and including a first surface sheet, amoisture-absorbing layer consisting mainly of high moisture absorbentfibers, and a second surface sheet; b) the fibers in themoisture-absorbing layer adhering to each other, and themoisture-absorbing layer and the surface sheets adhering to each otherby a self-adhering activity of the high moisture absorbent fibersexpressed by water absorption and/or moisture; c) at least one of thesurface sheets having air permeability; d) the high moisture absorbent,nonwoven, fabric structure having a thickness not more than 1.5 mm; e)the high moisture absorbent, nonwoven, fabric structure having a watercontent of 20 to 45 weight %; and f) the high moisture absorbent,nonwoven, fabric structure averaging a moisture-absorbing rate of atleast 20 weight % under an absolute dry condition at 20° C. and 65%relative humidity after 5 hours.
 8. The drying agent of claim 7, whereinthe moisture-absorbing layer consists of high moisture absorbent fibersalone.
 9. The drying agent of claim 7, wherein the high moistureabsorbent fibers are high moisture absorbent fibers of a crosslinkedsodium acrylate.
 10. The drying agent of claim 7, wherein the fabricstructure has an apparent density of 0.05 to 0.8 g/cm³.